1. Field of the Invention
This invention relates to methods of modification of foamed plastic articles and, more particularly, to methods of applying modifying ingredients to a mass of open-celled polyurethane foam.
2. Description of the Prior Art
Previous methods of applying dyes, pigments and other modifying ingredients to open-celled polyurethane foam and other open-celled foams were based generally upon adhesion of the modifying ingredient to the surface of the foam, as by painting, or upon physical mixing of the modifying ingredient with the constituents prior to foam formation.
For example, polyurethane fabrics, not greater than about 3/32 inch thick, may be dyed by various methods well known in the art, such as rope dyeing, or pigments and/or inert fillers may be added to one or more of the reactants before they are mixed in the mixing head of the well-known "one shot" reaction system commonly used in continuous polyurethane foam slab production.
One major disadvantage attached to foams produced from constituents compounded with modifying ingredients in the dilution of the characteristics of the modifying ingredient experienced during the expansion of the reaction mixture into foam. For example, if a pigment is mixed with one or more of the reactants before foaming, the foamed product will exhibit a lighter shade of color than would be imparted by direct application of the pigment to the foam's surface. It is extremely difficult to control the color of the final product in this situation.
It may be noted that previous attempts at dyeing a mass of foam by immersion, as by kettle dyeing, when the thickness of the mass is greater than about 3/32 inch, have been unsuccessful due to the inability of the foam mass to dry at a uniform rate throughout. This is due to the natural insulating properties of the foam combined with the tendency of open-celled foam to retain relatively large amounts of liquid in its cells. The added weight of the retained liquid is another factor that has made immersion dyeing of foams impractical. In foams comprising a fabric whose thickness is less than about 3/32 inch, this problem is not great as vaporization is not effectively hindered. Only when the thickness of the foam is less than about 3/32 inch will effective drying occur so as to allow profitable production.
Another adhesion method of coloring polyurethane foam is spray painting of the foam article, such as a grill cloth for a speaker system. This is accomplished by directly spraying paint onto the surface of the foam by means of conventional paint sprayers. This method is unsatisfactory due to a lack of penetration of the paint into the foam and undesirable changes in the surface characteristics of the foam.
Previous methods of treating foam generally required relatively high setup costs, thereby making short production runs unprofitable. This has proven inconvenient for producers whose customers desire relatively small quantities of treated foam.